The free piston Stirling engine has characteristics which make it particularly suitable and advantageous for use in many applications. Such engines are capable of driving a variety of loads and commonly are used to drive linear alternators so that heat energy from the combustion of fuels or from the sun can be used to generate electrical energy.
Typically the engine is designed to operate at a selected operating temperature and to supply a selected operating or maximum load power. For example, the engine may be designed to drive a linear alternator which supplies an electrical load. So long as the power demand of the electrical load remains constant at the design value, the free piston Stirling engine, which is an oscillator, remains in dynamic equilibrium and operates at the design output power, stroke amplitude and temperature.
Problems arise, however, when the equilibrium conditions are changed, for example by a reduction in the power demand of the electrical load. This reduction may be the result of reduced work demand or disconnection of the electrical load. If the engine is not provided with any power regulation, a reduction in the power demand of the alternator or other load on the engine will cause the strokes of the power piston and the displacer of the Stirling engine to increase. With insufficient load connected to absorb the excess available output power, the piston excursion amplitudes will increase until this "runaway" causes the piston and displacer to collide with each other and/or collide with other parts within the Stirling engine resulting in damage or destruction of the Stirling engine.
FIG. 4 illustrates the problem. FIG. 4 is a graph of Stirling engine power output versus piston displacement for a conventional engine. A Stirling engine operating at temperature T.sub.1 will exhibit a power out versus displacement characteristic curve T.sub.1. If the engine is connected to a load, such as a linear alternator, the load will have a characteristic curve illustrated as L.sub.1, which may, for example, be the design or maximum load on the alternator.
If the unregulated engine is started and an electrical load is supplied from the alternator, the piston stroke or maximum excursion amplitude will increase until equilibrium is reached at operating point O.sub.1. If the power output demand is reduced delta P while engine temperature remains at T.sub.1, the piston displacement will continue increasing because the excess energy will not be absorbed by the load. This instability causes a runaway condition because increased stroke results in even more unabsorbed energy output resulting in the ultimate damage or destruction of the Stirling engine and possibly the alternator.
If the engine temperature could be instantaneously reduced to temperature T.sub.2, then a new equilibrium operating point O.sub.2 could be reached at the reduced load L.sub.2. However, the mass of the Stirling engine prevents the instantaneous change of engine temperature and therefore under transient conditions, runaway will occur in an unregulated free piston Stirling engine.
A related problem occurs if a free piston Stirling engine is driving a load which undergoes a brief pause or interruption in its operation caused, for example, by a temporary overload. Under these conditions the engine oscillation may stop. Even a stop of short duration will cause the temperature of the engine to increase since the heat input energy is no longer being absorbed by the load or transferred to the cooler. When the engine restarts at a higher temperature, it will operate under a temperature curve which is higher than the temperature curve T.sub.1. Thus, a similar runaway condition will occur. Although the runaway condition may only be momentary, it may be sufficiently long that the engine will be damaged before its temperature can fall down to its design operating temperature T.sub.1.
Yet another problem is that the instability of the unregulated engine, which causes it to run away when there is no output power demand, requires that a Stirling engine either be started under load or started at a very low temperature in order to prevent immediate run away. Under load the engine is more difficult to start.
One solution of these problems is to provide an external variable load which absorbs the excess power when the power demand of the load is reduced. This is the subject of U.S. Pat. No. 4,642,547.
Yet another proposed solution to this problem is to electrically drive the displacer of the Stirling engine at a controlled excursion amplitude. In this system the displacer is driven by an electrical drive mechanism, typically a linear motor. The stroke of this linear motor drive is controlled by a control system. Displacer stroke is reduced when the power output demand is reduced and similarly is increased when the power output demand is increased.
The problem with this system is that it is far too complicated and expensive, requiring substantial control apparatus and additional external connections to the Stirling engine. This solution also exhibits transient problems since a finite time is required for such a system to respond to a variation in load power demand.